The present invention relates to an optical demultiplexing/multiplexing device which is used in an optical fiber communication circuit to demultiplex a light signal into a plurality of circuits or to multiplex a plurality of light signals into a single circuit, and especially to an optical demultiplexing/multiplexing device having flat spectral response for light signals consisting of a plurality of light wavelengths. It also relates to the simplification of the structure of the optical multiplexing device to demultiplex light signals from each of one or more optical fibers into a plurality of circuits, or to multiplex light signals from a plurality of optical fibers into one circuit.
FIG. 14 shows an example of a 2.times.2 optical demultiplexing/multiplexing device. In FIG. 14, "A" indicates the main body of the 2.times.2 optical demultiplexing/multiplexing device. In the optical demultiplexing/multiplexing device, the multiplexing section is composed of a pair of optical fiber elements 1 and 2. The optical multiplexing device is constructed by partly fusing the pair of optical fibers in accordance with the "fusing and drawing optical fibers" method, or by fusing the surfaces of the pair of optical fibers after they are polished in accordance with the "polishing the surfaces of optical fibers" method. The light signal passing through one optical fiber partly goes to the other optical fiber, and this type of optical coupling is accomplished by the Evanescent effect.
Light signal Ia passing through optical fiber element 1 branches into light signals Ib.sub.1 and Ib.sub.2 at junction 3. Since optical fiber 2 is unconnected at edge 2a, optical fiber 2 is cut at edge 2a and terminated there in such a manner that no light signal can reflect therefrom. The optical power ratio of light signal Ib.sub.2 branching into optical fiber 2 to light signal Ib.sub.1 passing through optical fiber 1 changes depending on the optical wavelength of the light signal if light signal Ia goes through junction 3 rightwards on an optical demultiplexing/multiplexing device actualized in accordance with the Evanescent effect. That is, the coupling coefficient of one optical fiber to the other changes in accordance with the optical signal wavelength. An optical demultiplexing/multiplexing circuit used to demultiplex an input light signal into more than two output light signals consists of two or more demultiplexing/multiplexing devices of this type, and variations of the optical power ratios from the defined value are added every time the demultiplexing/multiplexing device of this type is added to demultiplex an input light signal into two output light signals. This type of variations result in inferior uniformity in the optical output powers at the demultiplexing/multiplexing circuit outputs.
FIG. 15 shows another example of the 2.times.2 optical demultiplexing/multiplexing device, which can improve the uniformity in optical output powers at the demultiplexing/multiplexing circuit outputs. In FIG. 15, optical fiber elements 4 and 5 are sharply cut, sliced, and spliced at the lefthand edges thereof. The edges are thus combined to yield another optical fiber element, and the optical fiber is connected to optical fiber 6 at their lefthand edges.
This type of optical demultiplexing/multiplexing device wherein one optical fiber is connected to a pair of optical fiber elements whose connecting edges are sharply cut, sliced, and spliced at the lefthand edges thereof is disclosed in U.S. Pat. Nos. 4,666,541 and 4,720,161.
Alignment of the optical axes of the optical fiber elements, preparing the optical fiber elements, and working with these fiber elements cause problems if the teachings of these patents are used to construct an optical demultiplexing/multiplexing device.
FIG. 16 shows the configuration of a 1.times.8 optical demultiplexing/multiplexing device. The 1.times.8 optical demultiplexing/multiplexing device consists of seven 2.times.2 optical demultiplexing/multiplexing devices A1 through A7 and these 2.times.2 optical demultiplexing/multiplexing devices are connected together at junctions f1 and through f6.
The "fusion-and-splicing" method, wherein the optical fiber elements to be connected are spliced at the edges thereof to form a junction and fused there by a discharge arc, is mainly used for connecting one optical demultiplexing/multiplexing device to another.
An interconnection loss of 0.1 to 0.5 dB per junction occurs in optical fiber connections, and the interconnection losses are added for the optical demultiplexing/multiplexing device with multiple junctions. The demultiplexing/multiplexing device requires a plurality of connection parts, and the optical demultiplexing/multiplexing device with multiple junctions occupies a greater volume for the greater number of junctions.
In addition, the procedure for interconnecting a plurality of junctions in the respective 2.times.2 demultiplexing/multiplexing devices requires two or three minutes to fabricate a complete multiple junction device if the fusion-and-splicing method is employed to connect between optical fiber elements.
The inventor of the present invention has disclosed in U.S. Pat. No. 5,074,634 and No. 5,000,530 how to reduce the time needed for the troublesome procedure of fusing the interconnections of the respective 2.times.2 multiple junction devices.
Since an optical demultiplexing/multiplexing device with multiple junctions built in accordance with U.S. Pat. No. 5,074,634 and No. 5,000,530 is made of a plurality of 2.times.2 demultiplexing/multiplexing devices, the multiple junction device is expensive because each of the respective 2.times.2 demultiplexing devices are expensive. In addition, fabrication of an optical demultiplexing/multiplexing device with multiple junctions requires a greater number of processes including difficult steps. A device with multiple junctions furthermore occupies a greater volume.
The demultiplexing/multiplexing device with multiple junctions in the prior art containing many parts is expensive, and its response varies depending on the wavelength.
An objective of the present invention is to provide a demultiplexing/multiplexing device with multiple junctions, which can easily be built, contains a smaller number of parts and has a response which is flat over the specified wavelength range.
Another objective of the present invention is to provide an optical demultiplexing/multiplexing device of the star or tree type, which is made with only one optical connector, and which is characterized by ease of construction a smaller number of parts, and exhibits a flat response over the specified wavelength range. A further objective of the present invention is to provide an optical demultiplexing/multiplexing device which is suitable for demultiplexing and multiplexing of optical fibers of the ribbon type.